In wireless power transmission, energy is transmitted from a primary coil to a secondary coil by electromagnetic induction or magnetic resonance. Wireless power transmission systems are used in many fields, such as electric cars, mobile phones or other electrical equipment.
As there is a larger air gap between the primary coil and the secondary coil of a transformer (for example, in an electric car or a hybrid car, the primary coil is mounted on the ground while the secondary coil is mounted on the bottom of the car), the coupling coefficient is low and the leakage inductance is large, so a capacitor is used for compensating the leakage inductance of the transformer.
The compensation circuit topology commonly used is of SS type, in which a compensation capacitor is coupled in series with the primary coil and a compensation capacitor is coupled in series with the secondary coil, as illustrated in FIG. 1. A DC input voltage is accepted by the switch inverter circuit, and then an AC square wave voltage is outputted to a resonant network constituted by the primary compensation capacitor Cp and the primary inductor L1 of the transformer, and the energy is transmitted via transformer to a secondary resonant network constituted by the secondary compensation capacitor Cs and the secondary inductor L2 of the transformer, and then a DC output voltage is obtained at the output terminal after the rectifying circuit, so as to provide energy to the load. For the switch inverter circuit, a full bridge circuit consisting of S1, S2, S3, S4 as shown in FIG. 1 may be used, and other switch circuits, such as the half bridge circuit, may also be used. Controllable switch devices such as MOSFET (Metal-Oxide-Semiconductor Field Effect Transistor), IGBT (Insulating Gate Bipolar Transistor) or the like may be used as the switch of the circuit. Except that the rectifier bridge may be constituted by D1, D2, D3, D4 as shown in FIG. 1, the rectifier circuit constituted by controllable switch devices such as MOSFET, IGBT or the like may also be used as the rectifier bridge.
In a conventional device structure, the primary inductor L1 and the secondary inductor L2 of the transformer are packaged respectively in the transmission plate and the receiving plate which are both a closed container, so that the requirements for insulation, waterproof and dustproof may be satisfied. As shown in FIG. 2, the primary compensation capacitor Cp may be packaged in a separate container. Alternatively, as shown in FIG. 3, the primary compensation capacitor Cp may be placed in a cabinet together with the primary switch circuit. Similarly, as shown in FIG. 2, the secondary compensation capacitor Cs may be packaged in a separate container. Alternatively, as shown in FIG. 3, the secondary compensation capacitor Cs may be placed in a cabinet together with the secondary rectifying circuit.
When the wireless transmission system operates with large output power, the primary current and the secondary current will become very large as the power increases, so the inductor voltage and the compensation capacitor voltage on the primary side as well as those on the secondary side will become very large. In this case, there may be high voltages up to thousands of volts occurring on external terminals (i.e., terminals BC of the primary inductor L1 of the transformer, terminals EF of the secondary inductor L2 of the transformer, terminals AB of the primary compensation capacitor Cp, or terminals DE of the secondary compensation capacitor Cs) in the above described structure. Also, these high voltages may be transmitted for long-distance through wires, so there may be a safety risk.
In addition to SS compensation circuit, the compensation circuit topology may also be PS compensation circuit (in which a compensation capacitor is coupled in parallel with the primary coil and a compensation capacitor is coupled in series with the secondary coil, as shown in FIG. 4) and SP compensation circuit (in which a compensation capacitor is coupled in series with the primary coil and a compensation capacitor is coupled in parallel with the secondary coil, as shown in FIG. 5). The above problems may also occur at the series-coupled compensation capacitor side in the PS compensation circuit and in the SP compensation circuit, as shown in FIG. 4 and FIG. 5.
Accordingly, the issue needs to be solved for eliminating the above defects of the related art.